A primary step in the fabrication of semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of vapour precursors. One known technique for depositing a thin film on a substrate is chemical vapour deposition (CVD). In this technique, process gases are supplied to a process chamber housing the substrate and react to form a thin film over the surface of the substrate.
An example of a material commonly deposited on to a substrate is gallium nitride (GaN). GaN, and related material alloys (such as InGaN, AlGaN and InGaAlN) are compound semiconductors used for the manufacture of green, blue and white light emitting devices (such as LEDs and laser diodes) and power devices (such as HBTs and HEMTS). These compound semiconductors are usually formed using a form of CVD usually known as MOCVD (metal organic chemical vapour deposition). In overview, this process involves reacting together volatile organometallic sources of the group III metals Ga, In and/or Al, such as trimethyl gallium (TMG), trimethyl indium (TMI) and trimethyl aluminium (TMA), with ammonia at elevated temperatures to form thin films of material on wafers of a suitable substrate material (such as Si, SiC, sapphire or AlN). Hydrogen gas is generally also present, providing a carrier gas for the organometallic precursor and the other process gases.
Following the deposition process conducted within the process chamber, there is typically a residual amount of the gases supplied to the process chamber contained in the gas exhaust from the process chamber. Process gases such as ammonia and hydrogen are highly dangerous if exhausted to the atmosphere, and so in view of this, before the exhaust gas is vented to the atmosphere, abatement apparatus is often provided to treat the exhaust gas to convert the more hazardous components of the exhaust gas into species that can be readily removed from the exhaust gas, for example by conventional scrubbing, and/or can be safely exhausted to the atmosphere.
One known type of abatement apparatus is described in EP-A-0 819 887. This abatement apparatus comprises a combustion chamber having an exhaust gas combustion nozzle for receiving the exhaust gas to be treated. An annular combustion nozzle is provided outside the exhaust gas nozzle, and a gas mixture of a fuel and air is supplied to the annular combustion nozzle for forming a flame inside the combustion chamber for burning the exhaust gas received from the process chamber to destroy the harmful components of the exhaust gas.
This form of abatement apparatus is generally located downstream from a pumping system for drawing the exhaust gas from the process chamber. To prevent damage to the pumping system as the exhaust gas passes therethrough, a nitrogen purge gas is typically supplied to one or more purge ports of the pumping system for pumping with the exhaust gas. As a result, the gas received by the abatement apparatus usually also contains a significant amount of nitrogen.
Nitrogen is safe and requires no abatement. With the apparatus such as that described in EP-A-0 819 887, we have found that the destruction and removal efficiency (DRE) of hydrogen is very high, often exceeding 99.99%, whilst the DRE of ammonia is highly variable depending on the other gases contained within the exhaust gas entering the abatement apparatus. Ammonia is highly toxic, having a threshold limit value, or TLV, of 25 ppm, and we have found that the amount of ammonia exhaust from the abatement apparatus can be as high as 2400 ppm depending on the chemistry and the relative amounts of the gases contained within the exhaust gas.
It is an aim of at least the preferred embodiment of the present invention to seek to provide a method of, and apparatus for, combusting ammonia with a consistently high DRE irrespective of the other gases, and the relative amounts thereof, present in an exhaust gas containing the ammonia.